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An open channel flume with a central 180-degree bend with a rigid bed is designed to obtain a better understanding of the complex flow pattern around a T-shaped spur dike located in a sharp bend. The 3-dimensional velocities are measured by using an acoustic Doppler velocimetry under clear-water conditions. This study’s primary objective is to compare variations of the mean flow pattern along a 180-degree bend with a variety of T-shaped spur dike lengths. In order to do so, parameters such as streamlines, the maximum velocity distribution, and the secondary flow strength under the influence of three T-shaped spur dike lengths will be analyzed and then compared with the case where no spur dikes are implemented. The results show that with the spur dike placed at the bend apex, the mean secondary flow strength at that range increases by approximately 2.5 times. In addition, a 67% increase in the length of the wing and web of the spur dike leads to a 27% growth in the mean secondary flow strength along the bend.

The interaction between a forced vibrating pipeline and an erodible seabed in unidirectional flows is experimentally investigated. The experiments were conducted under clear-water scour conditions with a water depth of 0.3m and averaged approach velocity of 0.261 m/s. The pipeline model with a diameter (D) of 3.5 cm, was subjected to a vertical sinusoidal motion of varying amplitudes (A0 = 2∼6 cm) and frequencies (f0 = 0.1–0.6Hz). The initial gap (G0) between the lower pipe surface and the undisturbed flatbed level was fixed at 1D. The results show that the maximum scour depth increases significantly with both vibration amplitude and frequency, with the former having a more dominant influence. However, frequency tends to exacerbate the scour hole development more in the early scour stage. Distinct scour mechanisms are observed based on the different combinations of vibration amplitude and frequency. In the high A0 conditions (A0 > G0), the combined effect of pounding (when the pipe hits the seabed) and piston actions (rising and falling of the pipe) during the pipe descending period dominates the earlier stages of scouring, and the pick-up of sediment particles during its rising period controls the subsequent scour process. In the low A0 conditions (A0 < G0) when pounding does not occur, vortex shedding during the pipe-falling stage controls the development of scour hole when the frequency is low. When the frequency is high, however, the flow field associated with the pipe-rising period dominates the scour development. Different empirical formulas for the prediction of the maximum scour depth and width are proposed and compared in the present study. This work was financially supported by the National Natural Science Foundation of China (Grants No. 51779172, 51579175, and 51979186), and the Open Funds of State Key Laboratory of Hydraulic Engineering Simulation and Safety (Grant No. HESS-1708). The first author would like to acknowledge the support provided by China Scholarship Council (Grant No. 202006250062) for the one-year attachment at Nanyang Technological University (NTU).

This study experimentally investigates the mean and turbulent flow fields of a propeller jet impinging on a vertical quay wall. Four impingement distances, namely, wall clearances (= longitudinal distance between the propeller and vertical wall), were used to examine their influence on the flow behaviors. In order to investigate the characteristics of the three-dimensional impinging jet, both the streamwise (jet central plane) and transverse (impingement plane) flow fields were measured for each impingement distance using the particle image velocimetry (PIV) technique. Based on the wall clearance, three regions are identified, namely free, impingement and wall jet regions. Additionally, the results show that the evolution of the impinging jet is governed by two mechanisms: jet diffusion and wall obstruction. Their relative importance under varying wall clearances was interpreted in terms of the mean flow patterns and energy dissipation. Moreover, the proper orthogonal decomposition (POD) method is employed to identify the dominant flow structures associated with these two mechanisms in terms of the POD modes and their corresponding dominant frequency, which in turn provides a quantitative means to single out the individual flow mechanism. Nanyang Technological University The authors acknowledge the financial support provided by Nanyang Technological University for this research.

Local scour in a sediment bed varies depending on the type, size and shape of hydraulic structures, and the properties of approach flow and sediment particles. Published studies are hitherto confined primarily to empirical experiments based on particular types of hydraulic structure and there is almost no comparison of scour amongst different types of hydraulic structures. This study aims to provide an attempt to unify different types of clear-water scour based on a new length scale, which is proposed to characterize the size of the large-scale flow structure that governs scouring processes. The new length scale serves as a generalized hydraulic radius, which is applicable for all types of clear-water scour related to junction flows at bridge piers and abutments, and wall jets including culverts. The analysis indicates that when normalized with the new length scale, the equilibrium scour depths can be described with a unified function of the densimetric Froude number, regardless of different scour types. In addition, this study also shows that the proposed function varies with the scour efficiency, in a fashion resembling the bedload function in the range of low to high transport regimes. This finding, which is founded on solid physical grounds, reveals that different kinds of local scour phenomena can be interpreted in unison rather than based on the individual type of hydraulic structures. Published version The authors would gratefully acknowl-edge the financial support from the National Natural Science Foundation of China (Grant No. 51979242).

This study presents an experimental study on bed-load transport in the presence of emergent vegetation, which is simulated using uniformly sized circular cylinders arranged in a homogeneous distribution. The spatial variability of bed-load fluxes in vegetated flows is first examined. Then a model is proposed for evaluating the bed-load transport rate based on the scour holes that form around the vegetation stems. The influences of spatial variability and intensified turbulence on sediment transport are finally considered by relating the dimensionless transport rate and flow intensity to the stem size and vegetation density. The derived formula for predicting the bed-load transport rate in vegetated flows is applicable for weak to moderate transport conditions. Nanyang Technological University Published version The first author would gratefully acknowledge the financial support from the NTU Research Scholarship. The authors would also gratefully acknowl-edge the financial support from the National Natural Science Foundation of China (Grant No. 51979242) and tech-nical support from State Key Laboratory of Hydraulics and Mountain River Engineering at Sichuan University.

The bed-load transport rates predicted using Einstein's formula in the low and high transport regimes deviate from the experimental data. The deviation, which becomes significant when the transport rate is extremely low or high, may be solely attributed to the folded Gaussian distribution assumed by Einstein. The trends of an Einstein-type formula at the limits can be separately determined by considering the limiting behavior of the embedded probability distribution, which facilitates the derivation of a formula that best fits the bed-load data. By applying a Gamma distribution with the shape parameter equal to 3/2, a modified Einstein's formula is successfully applied to the low and high transport regimes. Nanyang Technological University Published version The first author would gratefully acknowledge the financial support from the NTU Research Scholarship. The authors would also gratefully acknowl-edge the financial support from the National Natural Science Foundation of China (Grant No. 51979242)

This study evaluates sediment gradation effects on clear-water scour at bridge piers. By considering the average erosion and deposition rates in a pier-scour hole, the normalized equilibriu...

This paper presents results obtained from a detailed analysis of the experimental data of three hydraulic jumps with low Froude numbers (1.5–1.9) in a narrow flume. With the increasing Frou...

This study presents measurements of the bed shear stress associated with the three-dimensional boundary layer induced by a wall-bounded swirling jet. The velocity profile within the viscous sublayer was measured in detail using a high-resolution particle image velocimetry system that enables direct evaluation of the bed shear stress from the velocity gradient at the wall. The three-dimensional flow field was reconstructed based on the measurements conducted in a series of intersecting streamwise and transverse planes. At each intersection, the resultant bed shear stress was calculated as the vector sum of its components. The results show that the planar distribution of the bed shear stress is highly asymmetrical about the propeller axis due to the swirling rotation, which furnishes a description of how the jet path migrates over the plane boundary under the swirling effect. Compared with the expected scour hole that the swirling jet could induce on an erodible bed, the results presented in this study shed light on a qualitative understanding of the distinctive jet impingement mechanisms associated with rigid and erodible bed boundaries. Moreover, a close examination of the mean velocity distributions of both individual and resultant velocities reveals a consistent discrepancy from the universal logarithmic law in the log layer, in which the velocity distribution is subjected to outer layer bulk flow properties, thereby highlighting the evident difference in boundary layer flow between an impinging jet and canonical wall-bounded flows.

Monopile foundations are the most common foundation type of offshore wind turbines. In the ocean environment, in addition to threats related to scour of the foundation, offshore monopiles of wind turbines also are subjected to lateral vibrations induced by waves and winds. Over the past decades, the scour process induced by flow and pile-soil interactions has been independently investigated by researchers from the fields of hydraulic/coastal engineering and soil mechanics, respectively. Very few studies can be found in the published literature on lateral vibration effects on scour at monopile foundations. The objective of this experimental study is to improve understanding of the mechanics of current-induced scour at monopiles subjected to lateral vibrations with a non-cohesive bed. When compared to a non-vibrating monopile, the results show that scouring around its vibrating counterpart is comparatively faster during the first few hours of the test. This is primarily due to soil densification and the subsidence process induced by monopile-seabed interactions. Increasing both the frequency and amplitude of the monopile vibration would decrease the equilibrium scour depths and scour hole slopes. The reduction of equilibrium scour depth likely is due to the effect of sediment ratcheting motions on the surface of the scour hole. An empirical dimensionless equation is proposed to estimate equilibrium scour depth at monopiles subjected to lateral vibrations. This research was supported by the Young Scientists Fund of the National Natural Science Foundation of China (51709082), the Fundamental Research Funds for the Central Universities (2018B13014), the National Science Fund for Distinguished Young Scholars of China (51425901) and 111 Project of China (B12032).

Submarine pipelines are widely used for the conveyance of hydrocarbons, oil or other fluids in the offshore industry. Although much progress has been made on pipeline-seabed interactions in the past decades, an experimental study solely devoted to investigating interactions between the flow fields, scour and forced vibrating pipelines in a quiescent water environment still has not been conducted. In practice, this condition, to a certain extent, resembles a Steel Catenary Riser (SCR) in deep offshore environments in the absence of currents and waves. The present study aims to obtain an improved understanding of the mechanics of scour around a forced vibrating pipeline in quiescent water. Tests with the same initial gap ratio G0/D = 1 and two different induced frequencies were conducted. A circular cylinder with a diameter of 3.5 cm was used as the pipeline model. With the help of a high-speed camera, the details of flows, pipeline motion and scour process are vividly presented in this paper. The magnitudes of flow velocity, vorticities, and turbulence fluctuations are found to be strongly dependent on the frequency of the vibrating pipeline. The scour mechanism of the two tests is different because of the complex interactions between the flow fields and pipeline motion. For the low-frequency case, the flow field associated with the falling stage of the vibrating pipeline controls the formation of the scour trench. On the other hand, for the high-frequency test, the scour process is controlled by the flow field during the rising stage of the vibrating pipeline.

Swirling jets induced by propellers have a significant impact on riverine and marine environments. Erosion around quay structures and contaminant dispersion in harbors or along navigation channels are examples of these effects. Simulating propeller-induced jet velocities that extend from near to far fields is the first step in elucidating this phenomenon and seeking solutions. Most previous investigations have proposed semiempirical equations for estimating velocity. Different formulas have been derived for velocity in both the zone of flow establishment and in the zone of established flow. In this study, the mean and turbulent flow fields downstream from a rotating propeller were investigated using an acoustic Doppler velocity profiler (ADVP). The results showed that the propeller-induced swirling effect was directly proportional to the propeller rotational speed, but decreased as the distance from the efflux plane increased. However, this effect has little effect on the radial distribution of the axial mean velocity and the decay of the maximum velocity. A point-source method was employed, along with the adjustment of the jet axial momentum spreading coefficient, to provide an analytical solution for the axial mean velocity within the propeller jet. Comparisons with measured data from the published literature and from the present study for both flat and equilibrium scour beds showed good agreement. Based on results of this study, the axial velocity profile calculated using the point-source method may be applied to investigate the scour and sediment resuspension induced by a propeller jet. Maritime and Port Authority of Singapore (MPA) Financial support provided by the Water and Environment Research Centre and Education Hub (DHI-NTU Centre), Hamburg Port Authority (HPA), and the Maritime and Port Authority of Singapore (MPA) to set up the experiment is gratefully acknowledged.

Wei, M.X.; Chiew, Y.M., and Guan, D.W., 2018. Flow behavior of Propeller Jet around an Open-Type Quay. In: Shim, J.-S.; Chun, I., and Lim, H.S. (eds.), Proceedings from the International Coastal Symposium (ICS) 2018 (Busan, Republic of Korea). Journal of Coastal Research, Special Issue No. 85, pp. 956–960. Coconut Creek (Florida), ISSN 0749-0208.The flow characteristics of a confined ship-propeller jet in the presence of an open-type quay slope with a rigid bed were experimentally studied using particle image velocimetry (PIV) technique. In the study, the toe clearance (Xt), namely the horizontal distance between the propeller and slope toe, was varied from 2Dp to 9Dp, where Dp = propeller diameter (= 7.5 cm). The propeller speed (n) and clearance height (Zb) were 500 rpm and 7.5 cm (= 1Dp), respectively. In each test, the field of view of the PIV measurement was focused on a rectangular region (∼30 cm × 20 cm) around the slope toe, which is the location of the greatest interest for engineering problems, such as scour damages in the case of an erodible seabed. The jet evolutions under different toe clearances are presented in the form of mean velocity and turbulence intensity from an overall perspective of the selected field of view. Due to the asymmetrical entrainment of the ambient fluid above and below the jet, observations reveal that the jet has a tendency to impinge onto the bed or quay slope. With increasing toe clearances, the jet undergoes significant diffusion before it reaches the slope. However, the turbulence intensity profiles show that the presence of slope causes a significant increase of turbulence in the near-bed region.

This paper aims to explore the combined main flow and turbulence influence on sediment pickup rates by investigating four purposely devised cases, namely, (1) unobstructed flow over a plane bed; (2) unobstructed flow over a rigid‐dune bed; (3) flow obstructed by a horizontal pipe; and (4) flow obstructed by a vertical pipe. The four cases were used to represent typical scenarios of mean flow combined with turbulence of varying intensities. By analyzing the dependence of sediment pickup rate on different combinations of the mean and turbulent flow parameters, a modified densimetric Froude number, incorporating both the bulk‐averaged velocity and maximum root‐mean‐square value of the fluctuating velocity, is found to be reasonably correlated with the measured pickup rates for all the four cases. Consequently, by relating the pickup rate with the dimensionless particle diameter and the modified densimetric Froude number, a generalized equation that predicts the pickup rates under both unobstructed as well as obstructed flow conditions is obtained. Published version

This paper presents an extensive review of the characteristics of a propeller jet-induced flow field and its impact on scour formation. As the driving force of scour, the jet flow behavior is first explored, including both the free and bounded jets under various boundary conditions. Accordingly, their resulting scour, namely, unconfined and confined scour, are expounded in terms of the temporal and spatial development of the scour depth. Hitherto, existing methods for predicting the scour depth are principally developed empirically, which exclude in-depth scrutiny into the mechanism governing the scouring process. With the detailed flow fields obtained in recent research, this paper aims to piece together a state-of-the-art understanding of the scouring mechanism governing confined propeller scour, that is, the combined effect of jet flow and junction flow on the scour hole development. By comparing the similarities and differences between the scouring mechanisms in open and closed quays, this paper provides a comprehensive insight into the scouring process associated with bounded propeller jets. Moreover, in terms of the near-bed flow variations during the scouring process, implications for sediment transport under highly turbulent flow are discussed. Finally, future research needs are identified. Published version

Although local scour around submarine pipelines has been extensively studied in the last few decades, understanding of the mechanism of local scour around pipelines is still in its infancy stage due to the complex nature of flow-pipeline-seabed interactions, especially when the pipeline is subjected to vibrations. This experimental study aims to obtain an improved perception of the scour mechanism around a pipeline subjected to vortex-induced vibrations. The experiments were conducted in a flow recirculation flume in clear-water scour conditions in which a circular cylinder with diameter (D) of 35 mm was used as the pipeline model. The initial gap (G0) between the underside of the pipeline and undisturbed flat-bed level was 0.45D. The time evolution of the pipeline motion and scour profile around the pipeline was measured using a high-speed camera and laser sources. The flow fields around the vibrating pipeline were obtained using the particle image velocimetry (PIV) technique and phase-average analysis. Based on the characteristics of the development of the scour hole and pipeline motions, three scour stages are identified. The downward motion of the vibrating pipeline and interactions between the counterclockwise vortex (downstream of the pipeline) that sheds from the lower shear layer of the pipeline and sediment bed are found to be the primary mechanisms that cause the formation of the scour holes beneath the pipeline. The interactive coupling effects among the vibrating pipeline, flow field, and scour process also are discussed in this study. Singapore Maritime Institute (SMI) The authors gratefully acknowledge the financial support provided by Singapore Maritime Institute (SMI) and EMAS AMC, under the SMI Deepwater Technology R&D Programme (Research Grant No. SMI-2014-OF-12). The first author also acknowledges support provided by the Young Scientists Fund of the National Natural Science Foundation of China (51709082) and the Fundamental Research Funds for the Central Universities (2018B13014).

An experimental study was conducted to better understand the settling velocity of porous spherical particles. Tests of the settlement of porous particles with porosity, ϵ, ranging from 0.53 to 0.86 in glycerol-water mixtures in a range of Reynolds number from 1.9 to 24,470 were conducted. The study utilized particle tracking and image velocimetry (PTV and PIV) to digitize the settlement trajectory in order to provide additional insights on the flow field around the particle. Particle properties, namely mass, porosity, and permeability, and fluid properties, namely density and viscosity, can potentially affect the terminal settling velocity. Experimental results reveal that flow permeation through the porous particle significantly increases with increasing porosity when the latter exceeds 0.77, but the drag coefficient decreases with increasing porosity at the same Reynolds number when the latter is less than approximately 100. The terminal settling velocity of porous and solid spherical particles with equal mass in air and in liquids (settling medium) is compared independently. Concurrent permeability effects on pressure drag reduction and frictional drag increment leading to the respective increase and decrease of the terminal settling velocity of porous particles are discussed. Flow visualization around a falling particle shows that compared with solid particles, porous particles cause earlier flow separation and the formation of a wider wake at Re100. Examination of the flow field through PIV analysis reveals that increasing porosity increases the velocity and weakens the turbulence in the recirculating (wake) region. Ministry of National Development (MND) This material is based on research/work supported by the Singapore Ministry of National Development under the Land and Livability National Innovation Challenge (L2NIC) Award No. L2NICCFP1- 2013-3.

An experimental method is presented in this paper to facilitate visualization of the detailed flow fields and determination of the near-boundary shear and normal stresses within an equilibrium scour hole induced by a vibrating pipeline. This method involves the implementation of a pipeline vibration system in a straight flume, a time-resolved particle image velocimetry (PIV) system for pipeline displacement tracking and flow fields measurements. The displacement time-series of the vibrating pipeline are obtained by using the cross-correlation algorithms. The steps for processing raw particle laden images obtained by using the time-resolved PIV are described. The detailed instantaneous flow fields around the vibrating pipeline at different vibrating phases are calculated by using a multiple-time-interval cross-correlation algorithm to avoid displacement bias error in the flow regions with a large velocity gradient. By applying the wavelet transform technique, the captured images that have the same vibrating phase are accurately cataloged before the phase-averaged velocity fields are obtained. The key advantages of the flow measurement technique described in this paper are that it has a very high temporal and spatial resolution and can be simultaneously used to obtain the pipeline dynamics, flow fields, and near-boundary flow stresses. By using this technique, more in-depth studies of the 2-dimensional flow field in a complex environment, such as that around a vibrating pipeline, can be conducted to better understand the associated sophisticated scour mechanism.

Although weirs or dikes in the riverine and coastal environments are frequently overtopped, few studies have hitherto examined the evolution of the scour process downstream of these structures under the submerged condition. This paper presents an experimental investigation on time evolution of the scour process downstream of submerged weirs with a uniform coarse sand. The clear-water scour experiments were carried out in a tilting recirculation flume. Different flow intensities and overtopping ratios (approach flow depth/weir height) were adopted in the experiments. Experimental observations show that the scour hole downstream of submerged weirs develops very fast in the initial stage, before progressing at a decreasing rate and eventually reaching the equilibrium stage. The results show that an increase of the overtopping ratio or flowrate can generate larger scour depth and volume downstream of the weir. Moreover, geometrical similarity of the scour hole that formed downstream of the weir was observed in the tests. Finally, empirical equations for predicting scour hole geometrical evolutions downstream of the submerged weirs were presented. The results of this study are useful in the development of numerical/analytical models capable of estimating the scour depth downstream of weirs in the river or coastal areas, for which the overtopping conditions are present. Published version

This paper presents an experimental study on the characteristics of the propeller-induced flow field and its associated scour hole around a closed type quay (with a vertical quay wall). An &ldquo, oblique particle image velocimetry&rdquo, (OPIV) technique, which allows a concurrent measurement of the velocity field and scour profile, was employed in measuring the streamwise flow field (jet central plane) and the longitudinal centerline scour profile. The asymptotic scour profiles obtained in this study were compared with that induced by an unconfined propeller jet in the absence of any berthing structure, which demonstrates the critical role of the presence of the quay wall as an obstacle in shaping the scour profile under the condition of different wall clearances (i.e., longitudinal distance between propeller and wall). Moreover, by comparing the vortical structure within the asymptotic scour hole around the vertical quay wall with its counterpart in the case of an open quay (with a slope quay wall), the paper examines the effect of quay types on the formation of the vortex system and how it determines the geometrical characteristic of the final scour profile. Furthermore, the temporal development of the mean vorticity field and the vortex system are discussed in terms of their implications on the evolution of the scour hole. In particular, comparison of the circulation development of the observed vortices allows a better understanding of the vortex scouring mechanism. Energy spectra analysis reveals that at the vortex centers, their energy spectra distributions consistently follow the &minus, 5/3 law throughout the entire scouring process. As the scour process evolves, the turbulent energy associated with the near-bed vortex, which is responsible for scouring, is gradually reduced, especially for the small-scale eddies, indicating a contribution of the dissipated turbulent energy in excavating the scour hole. Finally, a comparison of the near-bed flow characteristics of the average kinetic energy (AKE), turbulent kinetic energy (TKE), and Reynolds shear stress (RSS) are also discussed in terms of their implications for the scour hole development.

Vortex-induced vibration (VIV) has been studied extensively and much attention has been devoted to the “lock-in” phenomenon, which is primarily responsible for fatigue damage to structures. Nevertheless, the behavior of the surrounding flow can also be important. This paper investigates the flow characteristics of the wake in the initial branch for a vibrating cylinder using a new PIV technique capable of a high sampling rate of 200 Hz. The experiments were carried out over a wide range of reduced velocities to better understand the important trends. The mean velocities and turbulence characteristics were obtained by ensemble averaging repeated velocity measurements. This study reveals new insights on the flow characteristics of the wake in the initial branch. In particular, the cylinder vibration is found to lead to the formation of oblique jets, which have a profound influence on the mean flow velocity, turbulence intensity, formation and convection of the vortices, and variations to the stagnation and separation points of the cylinder during vibration. Accepted version

Although vortex-induced vibration (VIV) has been extensively studied, much of existing literature deals with uniform flow in the absence of a boundary. The VIV flow field of a structure close to a boundary generally remains unexplored, but it can have important engineering implications, such as pipeline scour if the boundary is an erodible seabed. In this paper, laboratory experiments are performed to investigate the flow characteristics of an elastically mounted circular cylinder undergoing VIV, and a rigid plane boundary is considered to simplify the problem. The initial gap-to-diameter ratio is fixed at 0.8, and six different reduced velocities are considered. The velocity field is measured using a high resolution particle image velocimetry (PIV) system, which has several advantages over traditional PIV systems, including high sampling rate and the ability to mitigate scatter of laser light near the boundary, allowing accurate measurements at the viscous sublayer. This paper presents the vibration amplitude and oscillation frequency for different Vr; in addition, the mean velocity field, turbulence characteristics, vortex behavior, gap flow velocity, and normal/shear stresses on the boundary were measured/calculated, leading to new insights on the flow field behavior.

Cyclic lateral loading on a monopile in windfarms has been found to cause bed subsidence and convective motions of sediment particles around its foundations. These soil deformations may affect the bearing capacity of the foundation and safety of the structure. This paper presents a 2-dimensional (2D) experimental study on the temporal evolution of bed subsidence and sediment convective-area around vibrating monopiles. The experiments were carried out in a sandbox with a transparent front wall, through which the temporal development of soil deformations around the vibrating monopile can be observed and recorded by using a high definition web camera. The characteristics of the soil subsidence and convective cells are examined with the recorded images, and the influence of the applied force and cyclic loading frequency on the soil deformation process are discussed. The experimental data show that increasing either the force or loading frequency can increase both the temporal development rates and the final size of the subsided bed and convective cells. Empirical equations for depicting the temporal evolution of the subsidence (depth and width) around monopile foundations are proposed.

Scour, which represents a fascinating and complex engineering problem involving a number of physical mechanisms and interactions, has motivated a vast amount of research following different but complementary methodological approaches such as experimental, numerical, and field methods. Far from being completely understood, scour remains one of the main hazards for many structures such as bridges, submarine pipelines, offshore wind turbines, etc. Thus, scour is currently a very active field of research with important open questions that are unanswered and practical challenges to be tackled. This Special Issue aims to bridge knowledge gaps by collecting fourteen papers to provide a wide view of scour types and different research approaches, with innovative ideas and inferences.

This study presents results of the temporal development of propeller scour around a sloping bank. During the scouring process, the centerline scour profile along the propeller axis comprises a toe scour hole, a primary scour hole, or a combination of both. Their formation is dependent on the length of the toe clearances, which is defined as the longitudinal distance between the propeller face and toe of the slope. As a result, the overall maximum scour depth along the centerline profile may occur either in the primary or toe scour hole. The results show that the temporal development of the overall maximum scour depth for all the cases consistently exhibits an exponential form. An empirical method was obtained to estimate the maximum scour depth evolution throughout the scouring process.

By using particle image velocimetry techniques, this paper presents new insights on the turbulence structure and time-averaged flows in a rectangular cavity with large aspect ratios. The three cavity aspect ratios (L=D, where L = cavity length, D = cavity depth) used in this study are 9.9, 13.0, and 18.9. The time-averaged velocity fields and Reynolds shear stress distributions within the cavity show that the flow pattern and turbulence structures are strongly affected by the cavity aspect ratio. The quadrant dynamic analysis of velocity fluctuations on the cavity shear layers and cavity downstream edge for the three cavities is conducted. Considering all of the bursting events with hole size parameter, H = 0 in the measured planes, sweeps are found to have the highest probability of occurrence within the zone of the shear layer, and ejections dominate almost all of the areas within the cavity and a certain depth of flow immediately above the shear layer. For the highmagnitude events (H = 1), quadrant dominances are almost inverted for all of the measured planes. The quadrant shear stress distributions show that the opposing events are approximately balanced for these regions on the cavity shear layers and along the cavity downstream edge. Accepted version

Turbulent dispersion causes sediment particles to be transported from high concentration regions to low concentration regions and determines the concentration distribution of suspended sediment. In this study, a new turbulent dispersion model is proposed for large-scale flows with suspended sediment. Two Stokes numbers are used to describe the turbulent dispersion through the Schmidt number: (1) st ≡ τ p / τ f , where τ p is the particle response time and τ f the fluid turbulence time scale, and (2) st η ≡ τ p / τ η , where τ η is the Kolmogorov time scale. The former is used to account for interaction between large eddies and sediment particles, while the latter for small eddies. The new turbulent dispersion model is validated against experimental data available for open channel flows under a wide range of conditions for dilute flows: the volume concentration of suspended sediment could vary from 10 - 6 to 0.08 , st could vary from 2 × 10 - 3 to 8 and st η could vary from 2 × 10 - 2 to 60.

Since local scour at bridge piers in rivers and estuaries is a major cause of bridge failure, estimation of the maximum local scour depth is of great importance to hydraulic and coastal engineers. Although numerous studies that focus on scour-depth prediction have been done and published, understanding of the flow and turbulence characteristics of the horseshoe vortex that drives the scour mechanism in a developing scour hole still is immature. This study aims to quantify the detailed turbulent flow field in a developing clear-water scour hole at a circular pier using Particle Image Velocimetry (PIV). The distributions of velocity fields, turbulence intensities, and Reynolds shear stresses of the horseshoe vortex that form in front of the pier at different scour stages (t = 0, 0.5, 1, 12, 24, and 48 h) are presented in this paper. During scour development, the horseshoe vortex system was found to evolve from one initially small vortex to three vortices. The strength and size of the main vortex are found to increase with increasing scour depth. The regions of both the maximum turbulence intensity and Reynolds shear stress are found to form at a location upstream of the main vortex, where the large turbulent eddies have the highest possibility of occurrence. Results from this study not only provide new insight into the complex flow-sediment interaction at bridge piers, but also provide valuable experimental databases for advanced numerical simulations. The authors would like to thank Professors Robert Ettema and Subhasish Dey for their helpful discussions. Special thanks to the Associate Editor Professor Charles Melching for his thoughtful help in proofreading the manuscript. Anonymous reviewers are also acknowledged for their constructive comments. This research was supported by the Young Scientists Fund of the National Natural Science Foundation of China (51709082) and the Fundamental Research Funds for the Central Universities (2018B13014).

By using the particle image velocimetry (PIV) technique, experimental results of the propeller jet flow field within a developing scour hole around an open quay are presented. The experiments were conducted with four toe clearances (defined as the longitudinal distance between the propeller face and toe of the quay slope), which account for three distinct scour profiles corresponding to the three toe clearance fields, namely, near, intermediate, and far clearance fields. To better understand the underlying scour mechanisms associated with each field, the mean flow fields at selected time intervals (t = 0, 0.5, 2, 12, and 48 h) are presented in vector and streamline plots. The data provide an improved understanding of the development of the main jet flow and vortex that formed in front of the quay slope. The importance of both the main jet flow and vortex in terms of their contribution in shaping the final scour profile in each field is discussed. Moreover, the turbulence intensities and near-bed Reynolds stress, which are analyzed for the intermediate field, exhibited a declining trend with the development of the scour hole.

Two important issues need to be addressed when using lattice Boltzmann methods to simulate particulate flows: complex moving boundaries and near contact of two solid objects. This study introduces a new boundary treatment method for using lattice Boltzmann methods to study fluid-particle interaction problems in which solid objects are close to a wall or two solid objects are near contact. The new method does not need to use any additional lubrication correction to total hydrodynamic force, instead implementing the no-slip boundary condition by introducing an appropriate fictitious flow field outside the fluid domain. The new method was verified by simulating a spherical particle falling down in fluids with and without rebound. It is concluded that the proposed boundary treatment method can accurately simulate the fluid-particle interaction near a rigid wall and collision in fluid.

The flow properties of a propeller jet in the presence of a plane bed boundary were investigated using the particle image velocimetry technique. Three clearance heights, Z b = 2D p, D p, and 0.5D p, where D p = propeller diameter, were used to examine boundary effects on the development of the jet. In each case, the mean flow properties and turbulence characteristics were measured in a larger field of view than those used in past studies. Both the streamwise and transverse flow fields were measured to obtain the three-dimensional characteristics of the propeller jet. Similar to a confined offset jet, the propeller jet also exhibits a wall attachment behavior when it is placed near a plane boundary. As a result, in contrast to its unconfined counterpart, the confined propeller jet features three regions, namely the free jet, impingement and wall jet regions. The study shows that the extent of each region varies under different clearance heights. The development of the mean flow and turbulence characteristics associated with varying clearance heights are compared to illustrate boundary effects in these regions. In the impingement region, the measured transverse flow fields provide new insights on the lateral motions induced by the impingement of the swirling jet. In the wall jet region, observations reveal that the jet behaves like a typical three-dimensional wall jet and its axial velocity profiles show good agreement with the classical wall jet similarity function.

A stochastic optimization model for urban drainage design (SODD) was proposed in this study to help analyze the tradeoff between investment and acceptable flood damage in urban drainage designs considering effects of both uncertainty and climate change. The simulation model (i.e. SWMM), driven by designed rainfall either from existing Intensity-Duration-Frequency (IDF) curves or future ones subjected to climate change conditions, was used to simulate flooding scenarios. The generalized uncertainty analysis estimation (GLUE) and Monte Carlo simulation methods were employed to quantify the system reliability which was adopted in the constraints of the optimization model. The results from a case study showed that the deterministic optimization was computationally efficient with no randomness encountered in hydrological simulation, although its solution was hardly reliable in achieving the target for flood mitigation. The stochastic version, on the other hand, could offer richer information on system reliability and help managers make a more robust decision. The results also revealed that the rainfall extremes under the impact of climate change could significantly affect system investment. The proposed method is advantageous in facilitating cost-effective planning towards a risk-based drainage design in light of various complexities.

This study experimentally investigates the temporal and spatial development of the scour hole induced by a confined propeller jet in the presence of an open type quay. The longitudinal dist...

Bridge-pier scour in alluvial rivers often occurs in live-bed conditions with dune migration. The scour depth attains its deepest value when the dune trough reaches the pier, followed by de...

Although many papers have reported suction influence on sediment transport, only few of them considered effects of suction zone length on sediment transport rate. In this study, experiments were conducted to investigate suction effects on sediment transport rate in a horizontal conduit with a suction zone with variable lengths. The results show that at the same suction intensity in the form of suction velocity ratio, the sediment transport rate increases with suction zone length, while all the data collapse if suction intensity in the form of suction rate ratio is used. Moreover, velocities measured using particle image velocimetry are used to explain the effect of suction zone length on the transport rate. Finally, a modified densimetric Froude number involving the effect of suction rate in terms of changes to the near-bed streamwise velocity is developed on the basis of this finding. Accepted version

A new numerical model is developed to simulate and investigate scour beneath a vibrating pipe during the tunnel erosion stage. This study is motivated by the fact that existing numerical models are not able to properly simulate scour under a vibrating pipeline, and the underlying physical mechanisms are not well understood due to the complex fluid-structure-sediment interaction. The present model incorporates the hybrid fictitious domain-immersed boundary method into a recently developed rheology-based two-phase model. The present model is validated against published experiment results of flow beneath a vibrating pipeline near a rigid boundary and scour beneath a fixed pipe. The flow velocity at the gap and the scour profile beneath the pipe are generally well produced by the model. Subsequently, the proposed model is applied to simulate scour under a vibrating pipe with different vibration amplitudes and frequencies. Among other things, it is found that maximum pipe acceleration has a dominant effect on the underlying physics that induce scour, irrespective of the combination of the vibration amplitude and frequency. An explanation for this finding is proposed based on various quantitative simulated results.